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Being more specific: the remote control will send a pulsed beam of infra-red light from and LED, usually rapid pulses (around 36KHz - 40KHz) interrupted at a slower rate to carry the necessary data to the receiver. The pattern of turning the rapid pulses on and off conveys the data.
The photodiode in the receiver can operate in one of two ways, either producing a tiny voltage when the IR light falls on it or changing conduction when light falls on it. Either way, after suitable amplification and filtering to remove the high frequency pulses, the original data can be recovered.
The high frequency is used because it allows easy electronic filtering, without it there would be too much random IR light from other sources which would interfere with the data.
The remote has a LED that is designed to send a series of pulses at a fixed frequency. The key pressed is encoded in this pulses. The carrier frequency of the signal is important (that is why one remote works for one unit but does not work for another).
The pulses encode the user code and the button code in a manufactuer dependent manner (philips had the original standard but deviations are common). Universal remotes work only for standards compliant devices.
The pulses are detected by a photodiode but the device is actually a 3-terminal IC. It produces an output only when it recognizes the sender.
The output is processed by a microprocessor. Because of the modulation, the IR-photodiode is relatively insensitive to ambient light.
Guess what? A modern IR photo-detector IC works poorly as a simple on-off control for opening a door or something because it is designed to be sensitive only when it receives bursts of data. If it receives continuous high frequencies then it turns down its sensitivity because modern compact fluorescent light bulbs produce continuous high frequency IR.
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